Precipitation hardenable ferritic iron-chromium-titaniium alloys

ABSTRACT

A RANGE OF HIGH STRENGTH STAINLESS STEEL ALLOY COMPOSITIONS, RESISTANT TO CHLORIDE STRESS CORROSION CRACKING, AND WHICH CONTAIN ESSENTIALLY IRON, FROM ABOUT 7.0 TO ABOUT 20.0 WT. CHRONIUM, AND AN AMOUNT OF TITANIUM FROM A MINIMUM CORRESPONDING TO THE LIMIT OF ROOM TEMPERATURE SOLID SOLUBILITY OF TITANIUM IN THE IRON-CHROMIUM ALLOY UP TO ABOUT 5.5 WT. PCT. THESE ALLOYS CAN BE SOLUTION TREATED AND AGED TO PROVIDE A MICROSTRUCTURE HAVING   AN ALPHA FERRITE MATRIX CONTAINING A FINE DISPERSION OF SECOND PHASE PARTICLES (CHI OR LAVES PHASE).

July 17, 1973 BUCHANAN ET AL 3,746,586

PRECIPITATION HARDENABLE FERRITIC IRON-CHROMIUM-TITANIUM ALLOYS Filed March 29, 1971 4 Sheets-Sheet 1 L/,//00 Cf /00%Fe \ZO FIG 2 Q Q Q J 20 o 09 Of +b Q a I x 1. 0c v-X 5.5 3.0 Canvas/1mm games of Invent/on 20 b /6.0 2'0 E M/VE/VTURS- DWARD R. BUCHANAN I WEIGHT PER CENT Cr LEMUE-L A MESH/5 THE IR ATTORNEY y 7,1973 E. R. BUCHANAN ETAL 3,746,586

PRECIPITATION HARDENABLEFERRITIC IRON-CHROMIUM-TITANIUM ALLOYS 4 Sheets-Sheet 2 Filed March 29, 1971 FIG. 3

/N VE/V TORS EDWARD R. BUCHANAN LEMUEL A. TAI-PSH/S THE//-? ATTORNEY July 17, 1973 E. R. BUCHANAN ETAL 3,746,586

PRECIPITATION HARDENABLE FERRITIC IRON-CHROMIUM-TITANIUM ALLOYS Filed March 29, 1971 4 Sheets-Sheet 3 FIG. 5

IN VEN T 0R5:

EDWARD R BUCHANAN LEMUEL A. TARSH/S THE/R AT T ORNE Y July '17, 1973 U N ET'AL 3,746,586

PRECIPITATION HARDENABLE FERRITIC IRON-CHROMIUM-TITANIUM ALLOYS Filed March 29, 1971 4 Sheets-Sheet 4 Solution treated, fol/owed by aging /0 hours at 600 "6.

Hardness Profile Cast Fe--/6Cr47'/' 1 E 1 t 58- As east Solut/on treated only one hour at temperature shown and water quenched to room temperature. (f l l 1 I l I 1000 I050 n00 //50 /200 /250 SOLUTION TEMPERATURE (6 Solution treated, fol/owed by aging /0 hours at 600 "6.

Hardness Profile Cast Fe-ZOCr-4 T/ a 62' As cast n 5.

.Solut/on treated only at temperature shown 54L and water quenched to room tempeature.

SOLUTION reupmarune (r) WVENTORS.

EDWARD R. BUCHANAN LEMUEL A. MRS/W5 THE/f? ATTORNEY United States Patent 3,746,586 PRECIPITATION I t ENABLE FERRITIC IRON-CHRONHUM-TITANIIUM ALLOYS Edward R. Buchanan, Burnt Hills, and Lemuel A. Tarshis,

Latham, N.Y., assiguors to General Electric Company, Schenectady, N.Y.

Filed Mar. 29, 1971, Ser. No. 129,036 Int. Cl. C22c 39/14 U.S. Cl. 148-37 5 Claims ABSTRACT OF THE DISCLOSURE This invention relates to precipitation-hardenable stainless steel alloys, and more particularly to ferritic stainless steel alloys containing essentially chromium and titanium in amounts to make them precipitation hardenable.

Cross-reference is made to co-pending application Ser. No. 103,598, entitled Ferritic Iron-Chromium-Titanium Alloys, filed Jan. 4, 1971, now abandoned in the names of E. R. Buchanan and L. A. Tarshis, assigned to the same assignee as the present invention, which describes alloys having a different percentage range composition and different microstructure, as compared to those of the present invention.

The class of alloys known as ferritic stainless steels are those made corrosion resistant by the addition of chromium in amounts generally greater than 12 wt. pct. The term ferritic refers to the body-centered cubic form of iron, also known as tat-phase iron. Ferritic stainless steels have good resistance to general corrosion and are also resistant to chloride stress corrosion cracking, but cannot be strengthened by precipitation hardening.

Heretofore, in order to achieve a combination of high strength and resistance to chloride stress corrosion cracking, users of stainless steel were forced to consider using either martensitic, precipitation hardenable, or duplex stainless steel alloys. However, the general corrosion resistance of martensitic stainless steels, by virtue of their lower chromium content, is poor relative to those with higher chromium content, thus limiting their use for many commercial applications. The duplex and precipitation hardenable classes of stainless steel are considerably more expensive than straight chromium ferritic alloys.

It is therefore an object of the present invention to provide a composition range of precipitation-hardenable ferritic stainless steels having yield and tensile strength prop-- erties competitive with known high strength stainless steel alloys.

Another object of the invention is to provide a high strength stainless steel alloy composition which is resistant to stress-corrosion attack.

A further object of the invention is to provide a class of high strength stainless steel alloys which is less expen- I SUMMARY OF THE INVENTION In accordance with these and other objects of the invention, a range of compositions is provided for stainless steel alloys containing essentially iron, from about 7.0 to about 20.0 wt. pct. chromium, and an amount of titanium from about 3.0 wt. pct., corresponding to the limit of room temperature solid solubility of titanium in the Fe-Cr alloy, up to about 5.5 wt. pct., the specific about of titanium being determined by the amount which can be added to the alloy without significant loss of engineering ductility. The microstructure of these alloys in equilibrium at room temperature is substantially a-ferrite phase with dispersed precipitates of chi and/or laves phases, except for the presence of small amounts of impurities existing as particles within the a-phase solid solution. These impurities include carbon and nitrogen present as TiC, TiN and Ti(C,N).

In contradistinction to most known commercial high strength stainless steels, the ferritic alloys of the present invention are resistant to stress corrosion, and they also have the capability of being strengthened by heat treatment to precipitate a second phase from solid solution. Moreover, because of the relatively low content of alloying additions necessary and the intrinsic price of the ingredients, these alloys are less expensive than many commercial stainless steels currently used to achieve a combination of strength and corrosion resistance.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the following description taken in conjunction with the accompanying drawings wherein:

FIG. 1 is a perspective view of a 3-dimensional ternary equilibrium phase diagram of the iron-chromium-titanium system, with the vertical axis of the diagram corresponding to temperature in C.;

FIG. 2 is a 2-dimensional schematic fragmentary representation of the iron-rich corner portion of a horizontal planar section of the prism of FIG. 1 taken along the horizontal plane located at 1000 C., in the direction of the arrows 22-2 of FIG. 1;

FIG. 3 is an electron micrograph (15,000 illustrating the microstructure of an iron-base alloy according to the invention containing 7.0 w./o. chromium and 5.5 w./o. titanium, after heat treatment to solutionize and precipitation harden the alloy, showing a dispersion of laves (7) phase precipitate;

FIG. 4 is a photomicrograph (200x) illustrating the microstructure of an as-cast specimen of an iron-base alloy according to the invention containing 16 w./o. chromium and 4 w./o. titanium showing coarse particles of chi phase in an alpha-ferrite matrix;

FIG. 5 is an electron micrograph (15,000 of an alloy having a composition similar to that of FIGS. 4 and after heat treatment to solutionize the chi phase;

FIG. 6 is an electron micrograph (15,000 of an alloy having a composition similar to that of FIG. 4 and 5, but after heat treatment to solutionize and also to age the alloy, and illustrating chi phase precipitate;

FIG. 7 is a plot showing two curves illustrating the effect on hardness of solution treatment temperature and also followed by aging, in a cast alloy containing 16 W./o. chromium and 4 w./o. titanium;

FIG. 8 is a plot showing two curves illustrating the effect on hardness of solution treatment temperature and also followed by aging, in a cast alloy containing 20 w./o. chromium and 4 w./o. titanium.

In the drawings, FIG. 1 is a representation of a 3- dimensional ternary phase diagram showing a plot of temperature in C. in the vertical direction, with the corner in the foreground representing percent iron, increasing amounts of chromium toward the right and increasing amounts of titanium toward the left.

In FIG. 2, a planar horizontal section of FIG. 1 is represented at the level corresponding to 1000 C. of the phase diagram.

In the phase diagram of FIG. 2, the curve designated by the letter L is the intersection of the 1000 C. temperature plane (222 of FIG. 1) with the solvus surface. The term solvus is defined as the locus of points representing the temperatures at which the various compositions of the solid phases coexist in equilibrium with other solid phases; that is, the limits of solid solubility. The curve designated by the letter L in FIG. 2 thus corresponds to the limit of solid solubility of titanium in the iron-chromium matrix at the temperature of this planar section, namely 1000 C. At room temperature the solid solubility of titanium in the iron-chromium matrix is about 3.0 weight percent.

The range of compositions of the percent invention is approximately indicated by the cross-hatch area in FIG. 2, from about 3.0 to about 5.5 w./o. titanium and from about 7.0 to about 20.0 w./o. Cr. However, since the curve L of FIG. 2 changes location with temperature, the various percentages of phases present in the microstructure of a respective composition may vary somewhat from that of FIG. 2. It should also be noted that at low chromium concentrations (less than about 10 W./o.) the amount of titanium which can be put into solution in Fe-Cr alloys is greater than at higher chromium levels. Whereas the limit of titanium solubility at room tem perature is about 3 w./o., for chromium concentrations greater than about 10 w./o., the Ti concentration may be as much as 4 w./o. at Cr concentrations of from 7 to 10 w./o. An understanding of this latter fact is necessary to explain, e.g. in Example H below, why compositions containing 3.3 w./o. Ti can contain essentially all aferrite.

The solubility of titanium in a particular iron-chromium alloy varies with temperature. The upper limit of titanium content in the alloys of the present invention corresponds to the amount which can be added such that a solution-treated and aged specimen will still have usable engineering ductility, e.g., greater than about 5 to percent tensile elongation at room temperature after heat treatment. This upper limit thus amounts to about 5 .5 w./o. titanium. Laarger amounts of titanium tend to cause brittleness in heat treated specimens after solutionizing and aging.

When the chromium content is less than about 8 percent, the second phase in the microstructure is predominantly laves (7) phase, as may be seen in the phase diagram of FIG. 2. Laves phase has an atomic composition approximately equal to Ti(Fe,Cr) Above about 8 percent chromium, the second phase is predominantly chi phase. Both the laves phase and chi phase cause the alloys to have age hardening capability.

The effects of the laves phase and chi phase will be discussed in connection with different alloy compositions.

The laves phase appears, under equilibrium conditions, at room temperature, in an iron-base alloy containing 7.0 w./o. chromium and about 5.5 w./o. titanium. In this alloy the structure consists substantially of an a-ferrite matrix plus a dispersion in the matrix of an intermetallic phase, i.e. the laves phase, designated in FIG. 2 as gamma (7). This structure corresponds to the zone in the phase diagram FIG. 2 identified as sq-kw.

FIG. 3 is a photomicrograph of a typical specimen having such microstructure, after solutionizing at 1200 C. followed by aging at 600 C. The tat-ferrite background with the laves phase distributed therethrough is clearly visible.

At greater chromium content, however, the second phase is chi phase rather than laves phase. Under equilib rium conditions, at room temperature, an iron-base alloy containing 16 wt. percent chromium and 4 wt. percent titanium consists substantially of an a-ferrite martix plus a dispersion in the matrix of the intermetallic phase designated chi The atomic composition of this chi phase is approximately Fe Cr Ti This chi phase in as-cast ingots is usually distributed as coarse particles, as may be seen in FIG. 4.

Although the room temperature strength and ductility of tests specimens prepared from as-cast ingots are poor, these alloys may be precipitation hardened.

The solutionizing heat treat temperature at which the alloys are heated in order to place the second phase (chi or laves) into solid solution, under equilibrium conditions, is the temprature at which the composition line of the phase diagram of FIG. 2 intersects the solvus line, L. Subsequent aging or precipitation hardening heat treatment is accomplished by reheating the quenched solutionized alloy to a temperature below the solutionizing temperature. By heating the workpiece to a temperature equal to or greater than that corresponding to the solvus line L of FIG. 2 (approximately 700 C.), the chi or laves phase can be dissolved in the a-ferrite matrix, thus providing (above about 700 C.) a single phase solid solution alloy. Rapid cooling (water quench) to room temperature, from the solvus temperature, causes sup pression of the precipitation of the second phase (chi or laves) particles. The tat-ferrite matrix is thus supersaturated with dissolved second phase. The alloy is then reheated to some intermediate temperature less than the solutionizing temperature. The precipitation of chi or laves phase during this reheating or aging step takes place as a fine dispersion of second phase particles. FIG. 3 shows laves phase as the second phase when the composition is low in chromium; and FIG. 6 shows chi phase as the second phase. Example I illustrates the effect of age hardening on the microstructure of one alloy composition according to the invention.

EXAMPLE I As-cast specimens were prepared of a composition containing nominally 16.0 wt. pct. chromium, 4.0 wt. pct. titanium, balance essentially iron, from high purity (99.9+%) iron, chromium and titanium, respectively. The composition was melted and cast into a 1%" diameter, 4" long copper mold having a 1%" diameter graphite hot top. A specimen cut from the ingot was heated in argon to 1050 C. and hot draw forged, in three successive workings with intermediate reheating to 1050 C., to a thickness of about A", and then hot rolled in three passes to approximately .065, with the finish roll temperature being approximately 850 C. The hot working was done to help refine the grain structure and improve chemical homogeneity.

The microstructure of the as-cast specimen is illustrated in FIG. 4 (200x The matrix may be seen as the lighter background, in which are embedded particles of the very hard chi phase, which contributes to the strength and hardness of the alloys of the invention.

In order to illustrate the capability of these alloys to be precipitation hardened, a series of as-cast specimens were solution-heat treated at various temperatures to place at least a portion of the second phase into solid solution, and then precipitated out in a finer dispersion by aging at a temperature lower than the solution temperature.

FIG. 5 is a photornicrograph (15,000X) of an ascast specimen having a nominal composition of 16.0 wt. pct. Cr, 4.0 wt. pct. Ti, balance essentially iron, solutionized by heating to 1200 C. for one hour and quenched in water to room temperature. The uniformity of grain structure and lack of dispersed particles shows that the chi phase has gone into solid solution in the a matrix.

FIG. 6 is an electron micrograph (15,000X) of a specimen of the same composition (16.0 w./o. Cr, 4.0 w./o. Ti) after it has been fully heat treated, first solutionized by heating to 1200 C. for one hour toplace the chi phase into solid solution, water quenched to room temperature and heat treated at 600 C. for 10 hours, to uniformly precipitate out the chi phase as a dispersion. Although FIG. 6 is at a much higher magnification, as compared to FIG. 4, the uniformity of appearance of the precipitated chi phase dispersion after the full solutionizing and precipitation heat treatment is clearly seen in FIG. 6 as compared to the coarseness and non-uniformity of size and configuration of chi particles in the as-cast condition shown in FIG. 4.

A comparison of FIGS. 6 and 4 shows that the particles of second phase, after solutionizing and aging, are much finer and more uniformly dispersed than the coarse particles of the same phase in the as-cast structure. The fine dispersion of the chi phase (FIG. 6) has a considerably greater capability for causing the a-ferrite matrix to resist plastic deformation than does the coarse dispersion in the as-cast alloy (FIG. 4). As a result, the precipitation hardened alloy having a microstructure of FIG. 6 is considerably harder and of greater strength than the same alloy before the precipitation hardening heat treatment. This is confirmed by the hardness graphs of FIGS. 7 and 8.

FIG. 7 contains two curves of hardness vs. temperature and pertains to an iron-base alloy containing 16.0 w./o. chromium and 4.0 w./o. titanium. The lower curve in FIG. 7 illustrates the effect on hardness, expressed as Rockwell A, of the solutionizing temperature. The lower curve thus illustrates that specimens are progressively softer when the 1-hour solutionizing treatment is conducted at a higher temperature, in the range of from 1025" to 1200 C., for this alloy; i.e. more of the harder chi phase is in solution at the higher temperature.

The upper curve of FIG. 7 illustrates the effect on hardness of the same solutionized specimens of the lower curve, subsequently aged, i.e. precipitation heat treated, for hours at 600 C. The two curves of FIG. 7 thus show that maximum hardness occurred in specimens heat treated by solutionizing for 1 hour at l200, water quenched to room temperature, followed by aging for 10 hours at 600 C.

FIG. 8 contains two curves and pertains to an alloy containing 20.0 w./o. chromium and 4.0 w./o. titanium. The lower curve of FIG. 8 illustrates the effect on hardness, expressed as Rockwell A, of solution heated specimens only, heated for one hour at one of the respective temperatures of the curve. It will be noted that for this alloy, at solutionizing temperatures above 1200 C., hardness of the solutionized specimens increases. Correspondingly, after the subescquent aging treatment of these same specimens at 600 C. for 10 hours, hardness dropped. Thus, maximum hardness in this alloy was achieved by solutionizing heat treatment at 1200 C. for 1 hour, followed by the aging treatment at 600 for 10 hours.

EXAMPLE II In order to demonstrate, by actual tensile tests, the strengthening capability of the precipitation hardening operation of the present invention, two pairs of alloys of different compositions according to the invention were processed and evaluated. Table I below lists the nominal compositions and the mechanical properties after processing to precipitation harden them according to the procedure following Table I.

l The tensile properites of Table I were determined at a strain rate of 0.02 in./iu./min.

Alloy A of pair I contains, in equilibrium at room temperature, before as well as after processing, essentially all alpha ferrite. Alloy B of pair I contains, in equilibrium at room temperature, two phases; namely, a matrix of essentially all alpha ferrite and a dispersion of a second phase which is laves phase: Ti(Fe,Cr)

Specimens of both alloys A and B of pair I were vacuurn induction melted, cast to /2" thick, 2" diameter buttons, forged to 0.220" thick at 1050 C. and hot rolled to 0.120" thick at 1050 C., an optional treatment to make the structure more uniform and refine the grain size. The specimens of pair I were then heat treated for one hour at 1200" C. to solutionize the structure Otf Alloy B, and water quenched to room temperature. They were rolled at room temperature to 0.060" thickness (optionally, to homogenize and refine the grain) and aged 10 hours at 600 C. Mechanical tensile properties for the specimens of pair I, tested at a strain rate of 0.02 in./in./ min., are shown in the upper portion of the right column of Table I.

The data of pair I show that in the compositions where the laves phase occurs as the second phase (e.g. alloy B), the alloys are capable of being strengthened by precipitation hardening to strengths about 5 0,000 p.s.i. greater than those of similarly treated specimen containing essentially all alpha ferrite (alloy A).

Alloy C of pair II contains, in equilibrium at room temperature, before as well as after processing, essentially all alpha rferrite. Alloy D of pair II contains, in equilibrium at room temperature, two phases; namely a matrix of essentially all alpha ferrite and a dispersion therein of a second phase which is chi phase, F17CTP7TI5- Specimens of pair II were treated according to the same schedule as described above for pair I. The mechanical properties of the specimens of pair II are listed in the lower part of the right column of Table I.

The data of pair II indicate that in the alloy compositions where the chi phase occurs as the second phase (e.g. alloy D), the alloys are also capable of being strengthened by precipitation hardening to more than 160,000 p.s.i., an increase of slightly less than 50,000 p.s.i. as compared to a similarly treated specimen containing all alpha ferrite structure (alloy C).

EXAMPLE III To demonstrate the stress-corrosion resistance of alloys of the present invention, several specimens of alloys according to the invention were immersed in 154 C. boiling magnesium chloride (MgCl .6H O). An austenitic stainless steel alloy (AISI type 347) of nominal composition: 18.0 w./o. Cr, 11.0 w./o. Ni, 1 w./o. Si (max.), .03 w./o. S (max.), .045 w./o. P (max.), 2.0 w./o. Mn (max.), .08 w./o. C, 0.8 w./o. Cb, was similarly treated. A comparison of U-bend specimens of the austenitic stainless steel and alloys of the present invention of composition Fe-16.0 Cr4.0 Ti showed extensive transcrystalline cracking in the austenitic stainless steel specimen after only 20 hours; whereas after 200 to 500 hours in boiling MgCl .6H O no cracking had occurred in specimens according to the present invention.

It will be understood, of course, that solution treatment and aging of the alloys of the present invention follow conventional age-hardening phenomenon. Accordingly, it is not necessary for all of the second phase to be dissolved in solution in order to obtain a particular age-hardening result.

Among the advantages of the alloys of the present invention is the fact that their compositions can be substantially free of nickel, thus making them less expensive than conventional stainless steels, with the added advantage that they are capable of being precipitation hardened and strengthened by heat treatment.

It will be obvious to those skilled in the art upon reading the foregoing disclosure that many modifications and alterations in the method steps and in the specific compositions may be made within the general context of the invention, and that numerous modifications, alternations and additions may be made thereto within the true spirit and scope of the invention as set forth in the appended claims.

What We claim as new and desire to secure by Letters Patent of the United States is:

1. A precipitation hardened stainless steel alloy consisting essentially in weight percent of from 7.0 to 20.0 percent chromium, greater than 3.0 percent and up to 5.5 percent titanium, and the balance being iron, said alloy being characterized by having a microstructure comprising a matrix phase of rat-ferrite and a dispersed phase selected from the group consisting of a laves phase having the formula Ti(Fe, Cr) and a chi phase having the formula Fe Cr Ti 2. The alloy according to claim 1, wherein the chromium content is less than about 8.0 percent and the dispersed phase is said laves phase.

3. The alloy according to claim 1, wherein the chromium content is greater than about 8.0 percent and the dispersed phase is said chi phase.

4. The alloy composition according to claim 1, said composition further containing as impurities, amounts from a trace up to amounts not in excess of 0.12 w./o. carbon, 1.00 w./o. manganese, 0.040 w./o. phosphorus, 0.030 w./o. sulphur and 1.00 w./o silicon 5. The body of claim 1, said body being characterized by having a stress corrosion resistance superior to that of AISI 347 austenitic stainless steel upon immersion in boiling (164 C.) magnesium chloride (MgCl .6H O).

References Cited UNITED STATES PATENTS 1,508,032 9/1924 Smith -124 2,801,916 8/1957 Harris 75-128 T 2,905,577 9/1959 Harris 75-126 T 3,065,067 11/1962 Aggen 75-124 3,251,683 5/1966 Hammond 148-37 3,359,094 12/1967 Bieber 148-37 3,365,343 1/1968 Vordahl 148-37 HYLAND BIZOT, Primary Examiner US. Cl. X.R. 75-126 D 

